1. Technical Field
The present invention relates to a method and apparatus for providing synchronization of an output signal to a reference signal, to be used for reference frequency synchronization in a mobile terminal, for example.
2. Description of the Related Art
Radio communication devices require generation of stable operating frequencies in order to function properly. Typically, stability has been obtained by using a crystal oscillator as a reference oscillator to provide a reference frequency. Specifically, local oscillators of radio terminals are phase-locked to the reference frequency. However, crystal oscillators by themselves cannot provide a sufficiently constant frequency to meet the frequency stability requirements of the radio terminal. In particular, the output frequency of a crystal oscillator varies over temperature. Additionally, non-linearities in the control path of the reference oscillator may cause frequency deviations.
Radio terminals may also require frequency correction to precisely center a radio transceiver operating frequency onto a counterpart station (e.g. base station) channel frequency. Frequency deviations may also occur due to Doppler shifts caused by movements of the terminal or due to frequency offsets at the counterpart stations (e.g. base stations). This is accomplished using an automatic frequency control (AFC) mechanism which determines an error between the radio transceiver operating frequency and the counterpart station (e.g. base station) channel frequency and applies a correction signal to the crystal oscillator to alter the reference frequency in order to synchronize the frequency of the radio terminal to the counterpart station (e.g. base station). The frequency error is typically obtained in a baseband circuitry where it is based on digital representation. A Digital-to-Analog-Converter (DAC) is used to obtain an analog control signal. This analog control signal is applied to a varactor diode which changes its capacitance in dependence of the applied voltage and therefore the frequency of the crystal oscillator changes, when the capacitance, that builds the resonance circuit varies. An extensive decoupling-network, consisting of several resistors and capacitors is used to obtain proper and especially low-noise performance.
This prior art solution has the disadvantage of additional cost and space and reduced reliability.
Moreover this analog control circuitry has typically no linear control characteristic.
In document WO 03/079548 a method is suggested to generate the exact local oscillator frequency on basis of an uncorrected reference oscillator by altering the division ratio of a Fractional-N-PLL (phase locked loop) circuitry that generates the local oscillator frequency on basis of the reference frequency of an uncorrected reference oscillator. However, in this solution, the exact reference frequency which is needed e.g. for processing of protocol timing and sampling of included Digital-to-Analog-Converters (DACs) and Analog-to-Digital-Converters (ADCs) cannot be provided in the mobile device.
Furthermore, in document U.S. Pat. No. 5,856,766, an exact local oscillator frequency is obtained by an apparatus on the basis of an uncorrected reference frequency by coupling both with a Fractional-N-PLL to which an information containing the initial frequency error of the local-oscillator is supplied to adapt the fractional division ratio in order to minimize the frequency error of the local oscillator. Further mixing or processing frequencies are obtained by providing further integer PLLs on basis of the exact local oscillator. This solution leads to the same problem, that the exact reference frequency itself is not generated, as mentioned above.
Thus, known frequency tracking mechanisms require corresponding control circuitry which involves costs and space, or do not supply any baseband or any output with the exact reference frequency.